You are here: Home / Project Results / Project Results

Project Results

Contents

VOC emission measurement results at aerobic wastewater treatment plant of mechanical pulp mill

 

VOC emission and effluent measurements were carried out at wastewater treatment plant of Stora Enso Anjala Mill in June - August in 2011. More measurements were carried out as planned because of the great variation in the VOC emissions and effluents.

In Stora Enso Anjala pulp mill the wastewater treatment system receives wastewaters from ten different points from the mechanical pulp mill. First phase in the plant is the clarifying and cooling the wastewaters. After wastewater has been cooled to about 37°C, wastewater will be pumped into the aerobic biological treatment. Flow-chart of the wastewater process is presented below.

Figure 1.1 Waste water treatment process flow.

Figure 1.2 Measured VOC emissions.

VOC concentrations of wastewater

VOC concentrations fluctuated greatly specially in the wastewater from the debarking plant where the fluctuation in different days may exceed the ratio of 1/5.  The fluctuation was also great in the wastewater from TMP plant (thermomechanical pulping). The concentration level was descending after the sedimentation and the cooling and also the fluctuation decreased but the distinct fluctuation was still observed. This also effected on VOC emissions.

Detected VOC compounds were mainly alcohols (ethanol and methanol) and some samples contained also turpentines. Additionally the samples contained several unidentified hydrocarbons.

VOC emissions released into air

The airborne VOC concentrations of aeration air of the wastewater basins caused fluctuation also in the hydrocarbon emissions into atmosphere between various measurement days. Especially in the MBBR basins the fluctuation was significant on different measurement days. The fluctuation of VOC concentrations was at highest threefold.

Detected VOC compounds were mainly same as in wastewater. Additionally also different turpentines origin from wood were detected from alcohols.

Unexpectedly, emissions also contained quite a high concentration of methane. Methane most probably originates from the anaerobic bacterial activity in waste water treatment system. In the additional measurements carried out on 26.10.2011 methane portion of VOC compounds was 24 v-% in the pre-sedimentation basin, 55 v-% MBBR1 basin and 40 v-% in MBBR2 reactor.

 

Figure 1.3 Specific VOC emission

VOC emission balances of the PGW and TMP plants were measured in the first VOCless Pulping project (LIFE06 ENV/FI/000201) in 2006-2009. Based on these measurement results the total annual VOC emissions from the PGW plant were 195 tons (production was 423.500 tons). The total annual VOC emissions from TMP plant were 41 tons (production was 87.000 tons). The total annual VOC emission balance of mechanical pulping production in Stora Enso Anjala Mill was about 236 tons in 2007.

In 2012 the total production in Stora Enso Anjala Pulp Mill was 500.000 tons. The total VOC emissions of the aerobic wastewater treatment plant were 445 tons in 2012. VOC concentration measurements of the aerobic wastewater treatment plant result the cleaning efficiency of 91%. Annually 17 tons of VOC emissions are released to the environment from wastewater treatment system.

 

VOC emission measurement results at anaerobic wastewater treatment plant of mechanical pulp mill

 

VOC emission and effluent measurements were carried out at anaerobic wastewater treatment plant of Kotkamills in May - August in 2012.

In Kotkamills the wastewater treatment system receives wastewaters from five different points from the pulp and paper mills. The system itself consists of two parts that have different principle of operation; aerobic and anaerobic wastewater treatment. The aerobic part has remarkable bigger volume, and over 80% of the mill’s wastewaters go through the aerobic wastewater treatment. Wastewaters that have remarkable high VOC-concentration (e.g. dirty condensate) go through the anaerobic wastewater treatment. In the reactor of the anaerobic part, methane is produced from the VOC-compounds of the wastewater. The methane is burned in lime sludge reburning kiln.

The wastewaters from the both treatment systems are lead to aeration basin, where the waters mix and odour and vent gases from the process are blown to the basin. Also VOC-compounds in the odour and vent gases are conducted to the basin.

The cooling tower of the aerobic treatment system is in use only on warm summer days. At other times there is no need for it. During the measurements the reactor of the anaerobic treatment system was not working properly and sometimes the system was passed by because of the malfunctions.

Figure 1.4 Waste water treatment process flow.

 

Figure 1.5 Measured VOC emissions

VOC concentrations of wastewaters

Effluent VOC concentrations vary highly over the time depending on the situation of the pulp production and operation of the wastewater treatment system. The strongest VOC concentration of wastewaters did origin from the anaerobic system (eg. likaislauhde).  The anaerobic reactor was not working properly all the time during the project measurements because of malfunction. This also affected the measurements and it was taken into account when planning the measurement activities.

VOC concentrations are equalized in the aeration basin where both the aerobic and anaerobic wastewaters are mixed. Measured VOC compounds were mainly alcohols (ethanol and methanol) and some samples contained also turpentines.

VOC emissions released into air

The VOC concentration in wastewaters was also detected in the hydrocarbon emissions into atmosphere in various measurement days.

Detected VOC compounds were mainly the same as the ones in wastewaters. In addition to alcohols also different turpentines origin from wood were detected. Unexpectedly, emissions also contained quite a high concentration of methane. It probably origins from the bottom of the sedimentation basin where anaerobic bacterial activity occurs.

 

Figure 1.6 Specific VOC emission

In Kotkamills there are more VOC emissions are dissolved in the wastewaters from the production (2,98 kg/produced pulp ton) than in Stora Enso Anjala Mill. 90% of the wastewaters will be treated in aerobic wastewater treatment system and 10% in the anaerobic reactor (strong concentrations). At the end all wastewaters are gathered to the aerobic treatment basin and from there led to the sea.

The total VOC emissions in wastewaters were 596 tons in 2012. Production was 200 000 tons. Based on the project measurement results the VOC cleaning efficiency in the aerobic basin was about 85%. In 2012 162 tons of VOC emissions were released into air after the anaerobic and aerobic wastewater treatment system at Kotkamills.

Cost efficiency calculations of piloted abatement techniques

 

The aim in the project was to provide a summary of experience gathered during the piloting periods of the three VOC abatement systems and to describe the performance of them. The conclusion report includes the description of the abatement systems, the feasibility and the performance of the systems.

The performance and the feasibility of tree abatement systems were tested in two mechanical pulping mills´ wastewater treatment plants of TMP and PGW pulping mills at Stora Enso Publication Papers Anjala Mill and TMP pulping mill of Kotkamills in Kotka, Finland.

The catalytic incinerator pilot plant was manufactured by Formia Emissions Control Oy (partner of the LIFE+ project). Biofilter system was manufactured by Meehanite (project coordinator). UV+ionisation filtration plant was manufactured by Desinfinator Oy (partner).

The efficient functioning of the pilot plants were assured by continuous monitoring, weekly visits and performance measurements. The technical monitoring and performance measurements were carried out by AX Consulting and Meehanite. The piloting period of each pilot plant was 2 – 3 months in each pulping plant. In addition, the following parameters were recorded with the help of continuous measurements: off-gas flow rate, inlet and outlet temperatures and pressure drop. Continuous monitoring of selected parameters took place via modem connection to a PC based office recorder at AX Consulting. The measurements of weekly visits contained inlet and outlet air temperatures, humidity and outlet air flow measurement and the correct run mode of pilot plants that were controlled as well as the sufficient irrigation of inlet gas and biofilter bed.

Additionally, standard short-term emission measurements and other control measures were carried out during the piloting period in order to insure that the cleaning efficiency of the VOC abatement system will stay stable. These measurements periods can be found in the measurement reports of each system and mill. The measurements of the cleaning and thermal efficiency levels of the VOC abatement system reveal that the pilot plant were functioning physically well and stable. The cleaning efficiency was sufficient only in the case of catalytic oxidation. Biofilter could reach the sufficient cleaning efficiency only when running in exceptionally long residence time. UV-filter could not meet the sufficient cleaning efficiency (> 60 %) in high concentrations in spite of several construction changes and pilot plant installations and running modes. In the range of low concentration (< 100 mgC/m3) the system reached sufficient performance. The method itself is cheap to install and run but the performance needs to be developed further.

The oxidation system turned out to be the best system by feasibility and performance. In addition the oxidation system was carefully simulated for the conditions to be met in aerobic and anaerobic wastewater treatment conditions. The results of the simulations are described in this report. The results illustrate the running cost of catalytic oxidation giving reliable cost estimations for abatement cases in real use. In the biofilter case the running cost are calculated on the basis of corresponding conditions in practise. This can be regarded accurate enough (+/- 10 %) because of the existence of normal running conditions for biofilter applications in pilot tests.  The VOC abatement systems that will be presented here are as follows:

Case study 1: Aeration exhaust air from the aerobic wastewater treatment plant of the moving bed bioreactor. The reason for these recommendations is at first the concentrations emitted. In aerobic wwtp the VOC emissions are usually under the set limit value of 50 mgC/m3. This was the case also in Anjalankoski. The presence of methane the MVOC-concentration varied on the level above 100 mgVOC/m3. In the other source of aerobic plant the limit value was not exceeded although the total VOC emission was much higher eg. in cooling tower air flow. When following strictly the set limit value only there is not an exact need to apply VOC-abatement in the aerobic wastewater treatment plant of Anjalankoski mechanical pulping mills. But the rule cannot be generalized because of the systems and air flows differ widely from pulp to pulp mill.

Case study 2: Vent gases of the anaerobic wastewater treatment plant of the anaerobic digestion stage. This full size anaerobic system represents Kotkamills full size anawwtp with the performance of the existing 1/10 wwtplant. In anaerobic wastewater treatment plant vent gases exceed the set limit value of 50mgC/m3 and the VOC abatement is recommended. In Kotkamills the concentration was on the level of 100 – 200 mgVOC/m3. In the beginning of the project it was not found a full size anaerobic wwtp who would have been interested to participate in project. It is known that in pulp mills of lower wastewater loads, like mills using recycled paper, have anawwtp in use. It is also understandable that there anawwtp is working properly with stable load from the processes. The variable load and other conditions caused many interruptions and shutdowns in Kotka.

Case study 3: Aeration exhaust air from Kotkamills aerobic aeration basin, which represents the combination of the hybrid wastewater treatment system and process VOC emissions. The case is abbreviated as “all Kotka”. The third case presented is because of the rationalisation of VOC emissions in mechanical pulp mill. The process emissions and wastewater emissions should be abated together in one and only abatement system. This makes the option most economically. This is why all VOC emissions are here handled with one abatement system in Kotkamills. 

The combinations presented in report Appendices 1, 2 and 3 are combinations of emissions sources and abatement systems. This combination is not the recommendation but the one presented in report chapter 5. The system diagrams in the appendices present the different emission sources and abatement systems piloted and measured in the project and the best options will arise in conclusion texts.

The production of pulp mass of 500 000 t/a in Anjalankoski (2011) and 200 000 t/a of Kotkamills (2012) have been stated and assumed to be the situation under emission measurements too.

Conclusions

Each calculation case is defined by the VOC concentration and airflow. The VOC concentrations over 50 mgC/m3 were noticed to abate due to the concentration level set up in the two VOCless projects (LIFE06 EN/FI/000201 and LIFE10 ENV/FI/568).

 

In table below the annual total costs of the regenerative catalytic oxidator and biofilter at AeroWWTP (aerobic wastewater treatment plant), AnaWWTP (anaerobic wastewater treatment plant) and “all Kotka” cases (hybrid wastewater treatment system and process VOC emissions) are concluded.

 

Annual total costs of catalytic incinerator and biofilter at AeroWWTP, AnaWWTP and “all Kotka” cases.

VOC emission source of mechanical pulping

and abatement system applied

Annual total cost of VOC abatement system

Increase of the cost of the produced pulp due to the use of VOC abatement system

Unit

€/a

€/t

AeroWWTP

 

Regenerative catalytic oxidator

60 000

0,12

Biofilter

36 000

0,07

AnaWWTP

 

Regenerative catalytic oxidator

68 000

0,34

Biofilter

42 000

0,21

“all Kotka”

 

 

Regenerative catalytic oxidator

77 000

0,39

Biofilter

143 000

0,71

 

 

Cost efficient abatement technology in AeroWWTP and AnaWWTP

The most cost-efficient abatement technology for the aerobic and anaerobic wastewater treatment emissions is biofilter. The low VOC concentration levels fits very well for biofiltration. The cleaning efficiency stays below 90 % and it is low compared to the oxidation of 96 – 99 %. The efficient cleaning system of catalytic oxidator is tested technology and fits tom the purpose but higher investment costs make the system 20 - 50 % more expensive. In case the source concentrations rises up to 500 mgC/m3 the oxidation is to be recommended.

 

Cost efficient abatement technology in “all Kotka” case

In the actual Kotka case of “all Kotka”, where all the remarkable VOC emissions are handled with aerobic aeration basin, like today, the case does not meet the limit value of 50 mgC/m3. In the case the VOC emissions staying in high concentrations (> 1 gC/m3) the oxidation system suits well. The higher the concentration is the more often the thermal oxidation becomes cheaper than catalytic one. In this case biofilter system has to be diluted with clean air and this makes the plant too big in size.